In the existing commercial sow populations, variation in postnatal growth performance of their offspring may be already preprogrammed before birth. Because these limitations in postnatal growth may only become apparent in the late grower and finisher stages, sorting pigs by weight at the nursery and grower stages will not resolve variation in growth performance that appears at the finishing stage.

Prenatal programming affects muscle development early in gestation. A low number of muscle fibers in low-birth-weight pigs limit their muscle mass (lean yield) at market weight. However, gut development and health status of low-birth-weight pigs are also affected by prenatal programming and reduce survivability through lactation and the nursery stages.

We have increasing evidence that the changing dynamics of prenatal survival in mature sow populations are resulting in increasing variance in grow-finish performance. It is likely that this is partly due to prenatal programming problems.

The critical questions for producers are, how do we recognize the problem of prenatal programming at the production level, and what can be done about it?

Muscle Development Before Birth

Select on Figure 1 to enlarge

The biphasic pattern of muscle fiber development (myogenesis) in the pig is illustrated in Figure 1.

In the first phase, Day 35 to Day 55 of gestation, a primary generation of so-called “primary myofibers” develops.

In the second phase, which lasts until Day 90 of gestation, the formation of secondary myofibers occurs. Over 20 secondary myofibers cluster around each primary myofiber. Considering the fact that an increase in muscle fiber number (muscle hyperplasia) ceases by around the 90th day of gestation, the number of primary and secondary muscle fibers formed by Day 90 ultimately determines the total number of muscle fibers at birth. Importantly, the total number of muscle fibers at birth is lower in smaller fetuses compared to larger fetuses.

In experimental studies, even modest levels of intrauterine crowding of embryos early in gestation have a negative impact on fetal muscle fiber development. Given the apparent imbalance in modern prolific sows between ovulation rate, early embryonic survival and uterine capacity — and consequences for fetal and postnatal development — the reproductive characteristics of prolific dam lines need careful consideration.

Although the primary goal of increasing the number of pigs born per litter may be achieved in prolific sow lines, associated adverse effects on prenatal programming are apparent. As a result, selection for increased litter size at birth has led to an increased between-litter variation in average piglet birth weight, as well as to an overall decrease in birth weight. In the extremes of high litter size born, the growth potential of the live born pigs that survive to weaning are seriously compromised by intrauterine competition with an increasing number of stillborn pigs that, obviously, never enter the nursery and grow-finish stages of production.

Birth Weight, Growth and Carcass Quality

Both the type and the total number of muscle fibers is fixed at birth and, between them, determine the lean growth potential of the pig. As the size and length of existing muscle fibers increases after birth, an increase in total muscle mass is apparent. Low-birth-weight pigs with low muscle fiber numbers, therefore, are expected to have impaired postnatal growth.

Experimental studies of within-litter variation in birth weight clearly demonstrate the associations among birth weight, carcass characteristics and meat quality traits.

For example, in the study shown in Table 1, pigs of low birth weight exhibited the lowest total number of muscle fibers, the largest muscle fiber size and the highest percentages of abnormal “giant” muscle fibers in the muscles investigated. These pigs also had the lowest percentages of muscle tissue, the lowest total protein and the lowest semitendinosus muscle weight, yet the percentages of internal organs, skin, bone and total water were highest, compared to their heavier littermates.